The present invention relates to tunable solid state laser systems, and more particularly to widely tunable systems employing a stable resonator.
Broadly tunable continuous solid-state lasers are attractive sources in medicine, atomic physics, spectroscopy and LIDAR. Popular materials for tunable sources included fluorides and chrysoberyl doped with chromium. The most widely tunable material known to date is titanium doped sapphire. Owing to very broad emission spectrum and excellent material properties, ti:sapphire is the most desirable material as a tunable source. Laser properties are less ideal because of a short 3.2 micro-second fluorescence lifetime but ti:sapphire alleviate this less desirable characteristic with a high emission cross section, excellent thermal conductivity and high melting point. First demonstration of titanium sapphire laser operation was by Peter Moulton in 1982. (P. F. Moulton, “Spectroscopic and laser characteristics of Ti:Al2O3,” J. Opt. Soc. Am. B3,133 (1986))
Since first operation, significant improvements in the growth process have substantially diminished reabsorption of the lasing mode. (Iain McKinnie, AnnMarie Oien, Don Warrington, Paul Tonga, Lawrence Gloster, and Terence King “Ti3+ Ion Concentration and Ti:Sapphire Laser Performance”, IEEE Journal of Quantum Electronics, Vol. 33, No. 7, July 1997) Improved figure of merit crystals resulted in lower thresholds and higher slope efficiencies by removing a parasitic Ti4+ ion from the sapphire crystalline host. (J Pinto, Leon Esterowitz, Gregg H. Rosenblatt, Milan Kokta, and Dennis Peressini “Improved Ti:Sapphire Laser Performance with New High Figure of Merit Crystals,” IEEE Journal of Quantum Electronics, Vol. 30 Nov. 1994 pg. 2612-2616)
A variety of methods have been employed for the production of a compact widely tunable source. In an arrangement by Hansch a z-folded cavity was end pumped using all lines of an argon ion source. A flat end mirror was mounted on a piezotranslator for rapid fine-tuning. For broad tuning a pair of microscope slides were mounted on a tiltable stage. Action of changing the relative spacing between the slides through an angular adjustment allowed tuning from 760-820 nm and 956-1010 nm with differing sets of mirrors. (C. Zimmermann, V. Vuletic, A. Hemmerich, L. Ricci, and T. W. Hansch “Design for a Compact tunable Ti:Sapphire Laser” Feb. 1, 1995/Vol. 20, No. 3/Optics Letters)
A low threshold CW ti:sapphire laser system was demonstrated by Moulton in an x-folded ring configuration using a frequency doubled Nd:YAG system. Low threshold values of 118 and 90 mW occurred with a beam waist of 25 and 16 microns respectively. In the respective publication threshold formalism was adapted and introduced for a four level system during continuous lasing. (James Harrison, Andrew Finch, David Rines, Glen Rines, and Peter Moulton “Low-Threshold, cw, all-solid-state Ti:Al2O3 laser” Apr. 15, 1991/Vol. 16, No. 8 Optics Letters)
      P    TH    =            η      c        ⁢                  π        ⁢                                  ⁢        hv                    4        ⁢        τσ              ⁢          (              T        +                  L          CAV                +                  L          XTL                    )        ⁢          (                        ω          o          2                +                  ω          p          2                    )        ⁢          1              (                  1          -                      ⅇ                          -                              (                                  α                  ⁢                                                                          ⁢                  l                                )                                                    )            
Similar low threshold systems were built for the production of mode-locked pulses using a z-folded cavity without need of a separated mode-locking element. Stable mode locking induced by the nonlinearity of the active medium was termed Kerr Lens-mode locking. A stable Kerr lens-mode-locked ti:sapphire system was demonstrated by Fujimoto using a 8-micron mode waist to produce CW operation threshold with 120 mW and KLM mode-locking with 156 mW of incident pump. (A. M. Kowalevicz, Jr., T. R. Schibli, F. X. Kartner, and J. G. Fujimoto “Ultralow-threshold Kerr-lens mode-locked Ti:Al2O3 Laser” Nov. 15, 2002/Vol. 27, No. 22/Optics Letters)
Multiple strategies have been employed for the process of pumping tunable solid-state media. A common method is to use an arc lamp or comparable light source to excite a laser rod. The laser rod and flash-lamp pump source are placed at the foci of an elliptically shaped reflector. Drawbacks of this method included poor spectral overlap between the absorption of the laser material and the emission of the laser rod as well as poor volumetric overlap and coupling loses at the surface of the rod and sides of the cavity.
Flash-lamp pumping techniques have been applied to Ti:Sapphire materials for the production of high-energy pulses. To overcome the lack of absorption efficiency the flash-lamp emission is converted via fluorescence conversion process into the blue-green emission region. (L. Esterwitz, R. Allen, and C. P. Khattak, in Tunable Solid State Lasers, Vol. 47 of Springer Series on Optical Sciences P. Hammerling, A. Budgor, and A. Pinto, eds. (Spinger-Verlag, Berlin, 1985), pp. 73-75.5) Further improvements were made on the flash-lamp configuration by switching from a coaxial to a linear flash-lamp. Attempts were made to improve the conversion efficiency by altering the content of the liquid dye surrounding the lasant material. (Philip Lacovaora, Leon Esterowitz, and Rodger Allen, Optics Letters, Vol. 10, No. 6 p. 273-275) Despite multiple attempts with Coumarin 504 and LD490 the most efficient solution remained Coumarin 480. Despite higher efficiencies than utilizing non-fluorescent cooling media, coumarin dye lacked stability. Decreases of 20% or more were reported after only a few hundred shots at 35 J.
Shortly after the introduction of highly efficient room temperature operated single emitters, diodes were implemented in end pumped solid-state laser configurations. In U.S. Pat. No. 3,982,201, Rosenkrantz et al. introduce a solid-state laser that is pumped by single diode lasers or arrays of diode lasers to which the solid-state laser rod is ‘butt coupled’ or end pumped. Early generations of diode arrays utilized low duty cycle operation to reduce thermal fluctuations allowing the diode to maintain spectral stability.
Increasing potential output power implementing single emitter and diode arrays lead to further developments in pump configurations. In U.S. Pat. No. 4,710,940, Sipes, Jr. introduces a Nd:YAG solid-state laser that is end-pumped by a diode laser array or by two diode laser arrays by combining the respective outputs through the usage of polarizing beam-splitting cubes. To account for ideal overlap behavior in such arrangements, Sipes, Jr., references the observation of D. G. Hall in “Optimum mode Size Criteria for Low Gain Laser,” Applied Optics, 579-583, vol. 20, (May 1, 1981), and reiterates the presumption “pump profile shape does not matter much as long as all the pump light falls within the resonator mode.” Sipes, Jr., states that to maintain ideal overlap and overcome the divergence problem one could implement cylindrical lenses mounted in the sagittal and tangential plains. Baer presents in U.S. Pat. No. 4,761,786, described a Q-switched, solid-state laser that is end-pumped by a single emitter diode laser or diode laser array. The invention is shown to collect the emitted light using collimating lens and focus the collimated beam directly into the end face of the rod. Baer discusses the possibility of correcting for astigmatism prior to the collimating and pump optics.
The aforementioned strategies are well known for coupling the output of high-power diode laser into solid-state materials. Differing divergences and beam quality in the sagittal and tangential plane are due to the asymmetry in spatial distribution of the diode emission area. Beam quality of a stable emitter is often correlated to the emitter width. The ideal beam presumed to have an emitter width comparable to the height. However, catastrophic optical damage leads to limitations in the width of the diode stripe. Typical values are 15-30 mW per micron width while heights are on the order of one micron. A commonly used combination of vertically layered materials used for the production of such devices is aluminum gallium arsenide (AlGaAs).
These so called broad-area lasers, with emitter widths greater than 5-10 microns often show a filamentary formation arising from nonlinear interactions between the laser field and the carrier signal within the active stripe. Upon reaching a population inversion, rapid increase in intra-cavity flux narrows the gain region by inducing in the central region of the stripe a higher refractive index. Contribution of the lower index modes decreases in response to the lateral variation of index. The lateral deviation in index across the profile of the stripe induces a guided effect in the diode stripe leading what is referred to as gain guiding. In a broad area single emitter diode the stripe will typically have an emission width greater than 10-15 microns in higher power emitters up to 400 microns. Within the lateral region multiple index-guided structures will form within the active area. Each index-guided region forms a filament, which is not spatially coherent with adjacent filaments. The filaments within the active material are the fundamental causes of spatial incoherence and are what leads to limitations in brightness.
A variety of methods have been described for generating laser output in the 532 nm wavelength range from solid-state lasers and diode lasers by utilizing the nonlinear process of second-harmonic generation (SHG). For example, several methods have been described for producing SHG laser output in the 520-540 nm wavelength range from diode-pumped solid-state lasers containing neodymium-doped host materials. Baer, et al. in U.S. Pat. No. 4,653,056 described a method in which an AlGaAs diode laser end-pumps solid-state laser resonator containing a Nd:YAG rod and potassium titanium phosphate (KTP) nonlinear crystal to produce SHG laser output at 532 nm.
Kozolvsky, et al. in their article “Efficient Second Harmonic Generation of a Diode-Laser-Pumped CW Nd:YAG Laser Using Monolithic MgO:LiNbO3 External Resonant Cavities,” (IEEE Journal of Quantum Electronics, vol. 24, No. 6 (June 1988)), describe producing about 30 mW of SHG output at 532 nm by using a diode-pumped Nd:YAG, single mode ring laser operating at 1064 nm to pump an external monolithic cavity of nonlinear magnesium oxide: lithium niobate (MgO:LiNbO3). Since Kozolvsky's publication methods of efficiently converting 1064 nm into 532 nm light via seconds harmonic generation have vastly improved. Brown et al. states that “efficiency from the 808 nm pump diode output to green output in the range of 10-30% for Nd:YVO4” in Brown et al. U.S. Pub. No.: US 2006/0098696.
Another example of a diode-pumped tunable laser is disclosed in U.S. Pat. No. 5,317,447 issued to Baird et al. The '447 patent teaches a high-power semiconductor diode laser or array of semiconductor diode lasers that optically end-pumps a compact, tunable laser with a pumping beam well-matched to the absorption bandwidth and mode volume of the solid-state laser. Tilted birefringent plates mounted in the solid-state resonator cavity control the spectral bandwidth and wavelength output of the wave-guide pumping beam. The diode pump laser of the '447 patent is said to produce an almost round beam with 610-690 nm wavelength range (well matched to the absorption of the tunable material).
Researchers are in need of a small footprint, low cost, broadly tunable laser source providing high brightness, intensity and excellent beam quality over wide range of wavelengths. Light sources such as these serve as a research tool in a variety of applications, for instance for the investigation of absorption and transmission of optical materials, biological samples and general chemical analysis, and others.
The present invention contemplates elimination of drawbacks associated with the known laser systems and provision of a tunable solid-state laser system capable of producing a TEM 00 diffraction limited tunable output using a high quality diffraction limited solid state pump source for end pumping.